I have an old Mac 10-10 that sits in my storage shed unused for years at a time. After 5 years of NO use , it recently started on the second pull .... two stroke oil is the only additive in the 5+ year-old gas.

I'm no chemist, and I don't even play one on TV - I'm just a cranky guy with a laptop and a bunch of gasoline powered equipment like the rest of you.

Well, pure-gas.org lists 166 stations in Oregon where currently you can buy non-ethanol gas. There's a fair chance that years ago there were even more that sold non-ethanol gas, and that may be what's in your saw. And yes, before the Great Satan ethanol, mixing 2-stroke oil with the gas was sufficient to maintain the volatility of my stored fuel for long periods as well. It does not appear to be sufficient now that we have only E-10 available here (CT).

Also, plastic fuel containers can "breathe" over time, allowing in moisture and passing out the fuel's volatility.

I've had good luck with Star tron (or Soltron - same stuff but more concentrated) as well as Sta-Bil (however, after 4 years in a generator, the fuel tank was dry and rusted and there was only a red deposit in the carb's float bowl). If you look on the label of a recent bottle of Sta-Bil, it mentions that the bottle has a 2 year shelf life once opened, and has a place for you to record the date that you opened it. That being the case, I imagine that 2 years would also be the maximum length of time that fuel with Sta-Bil would be useful.